The invention relates to an apparatus for generating rotary oscillations with two shafts pivotable relative to each other, wherein an end section of a first shaft is accommodated in an end section of a second shaft so as to form an air gap. Furthermore, the invention relates to a method for generating rotary oscillations.
The providing of rotary oscillations, i.e. timely varying rotary movements, is necessary for different applications, in particular in the fields of checking machines, devices and machine parts, which are subjected to a rotary movement.
Different apparatuses for generating rotary oscillations have already been known from the prior art. These are particularly apparatuses, wherein a constant rotary movement is superposed by a modulated pressurisation of a hydraulic coupling so that the sum of the two rotary movements will be provided at the outlet. Such pressure-modulated hydraulic couplings have been known, e.g. from DE 2 057 347 A, U.S. Pat. No. 4,283,957 A and U.S. Pat. No. 3,772,913 A. However, by means of such pressure-modulated hydraulic couplings only frequencies of up to 800 Hz at the most can be achieved due to the compressibility of the hydraulic fluid.
A different apparatus has been known from JP 2003126776 A, wherein two stationary magnets are provided which are fed by a direct-current source. Thus, such an apparatus only allows for generation of a torsion oscillation about a fixed zero position.
Furthermore, it has also already been known to generate rotary oscillations by means of electric machines. Here, a rotor of an electric machine is co-rotated with an electric rotary field, wherein both a basic rotation and a rotational uniformity are provided for achieving the rotary oscillations desired in the rotary field. In order to be able to keep the moment of inertia of the rotor low, synchronising machines are used in general. However, with such machines only rotary accelerations of up to 12,000 rad/s2 at the most can be achieved in the unstressed condition. If such machines are stressed with a test specimen for testing purposes, which has a moment of inertia of, e.g., about 0.1 kgm2, only rotary accelerations of up to 4,000 rad/s2 can be achieved with such synchronising machines.
It is an object of the present invention to create an apparatus for generating rotary oscillations, by means of which high-frequency rotary oscillations, in particular rotary accelerations of more than 10,000 rad/s2, can be generated in a simple manner.
This is achieved by an apparatus of the initially defined kind in that one of the two shafts comprises means for generating a magnetic field of substantially constant field strength, and the other shaft comprises conductors which generate a magnetic field, wherein current can be variably applied to said conductors for pivoting the shafts relative to each other. Compared to the synchronising machines known, high rotary accelerations of more than 10,000 rad/s2, in particular of more than 40,000 rad/s2, can be achieved in a simple manner by arranging the two shaft sections into one another and by generating a magnetic field by the aid of one of the two shafts, which magnetic field is timely substantially constant in strength and which, for generating a rotary oscillation movement, is superposed by the magnetic field generated by the conductors, to which current has been variably applied. Here, the field strength of the magnetic field, which is of substantially constant strength, may also be subject to certain changes, but these changes are smaller than those in the field strength of the magnetic field generated by the conductors, to which current has been variably applied.
For generating a magnetic field of substantially constant strength, by means of one of the two shafts, it is beneficial if conductors connected to a current source are provided as means for generating a magnetic field of substantially constant field strength. The magnetic field of timely substantially constant strength may be designed to be spatially constant or spatially rotating with respect to the shaft generating the magnetic field. For creating a magnetic field which spatially rotates with respect to the shaft generating the magnetic field, the conductors for generating the magnetic field of substantially constant field strength may be designed as multi-phase windings. Alternatively, it is also possible to provide permanent magnets as means for generating a constant magnetic field.
For checking machine parts which are subjected to rotary stress, it is advantageous if the two shafts are mounted in a freely rotatable manner, wherein the shaft designed as an input shaft is driven. Here, the input shaft is usually driven at a substantially constant number of revolutions such that the two shafts rotate at a substantially constant number of revolutions due to the magnetic field of substantially constant field strength; yet, for generating rotary oscillations, the magnetic field of substantially constant field strength may be superposed by a varying magnetic field by the aid of the conductor to which current can be applied variably, thus putting the drive shaft into rotary oscillation as a function of the variably applied current. With a special design of the apparatus, the input shaft may also be designed to be torque-proof and only the output shaft be rotatably mounted.
For driving the input shaft, it is beneficial if the input shaft is directly connected with a drive shaft of a driving unit, in particular of an electric motor. Alternatively, for driving the input shaft, it would also be conceivable to connect the input shaft with a drive shaft of a driving unit, in particular of an electric motor, via a torque-transmission device, e.g. via a drive belt.
As to an efficient transmission of the magnetic forces acting in the area of the shafts arranged one into another, it is advantageous if one of the shafts comprises an annular gap so as to form an air gap and for accommodating a cylindrical hollow-shaft section of the other shaft. Furthermore, as to an efficient transmission of forces, it is advantageous if the means for generating a magnetic field of substantially constant field strength, or the conductors, are arranged in the part of the shaft that is provided radially outside the annular gap. Here, it is particular beneficial if the input shaft comprises the annular gap for accommodating the cylindrical hollow-shaft section of the output shaft. Certainly, the annular gap could also be provided in the output shaft, a hollow-shaft section of the input shaft being accommodated by said annular gap.
In order to keep the moment of inertia of the shaft comprising the annular gap low, it is beneficial if the rotatable part arranged radially outside the annular gap is surrounded by a torque-proof stator so that the outer part of the shaft can be designed to be comparably thin-walled. Here, the magnetic flow may be guided from the outer part of the shaft into the stator via an air gap.
Moreover, it is preferred that the input shaft comprises the means for generating a magnetic field of substantially constant field strength and that the output shaft comprises the conductor windings to which current may be applied variably. Yet, it is also possible to generate a magnetic field of substantially constant field strength by means of the output shaft and to generate a timely varying magnetic field by means of the input shaft for generating rotary oscillations.
In order to be able to achieve as high a rotary acceleration as possible by means of the apparatus also at high frequencies, it is beneficial if a cage winding is arranged in the air gap between the two shafts. By the measure of providing a cage winding, inductance of the conductors generating a timely variable magnetic field can be kept low, thus achieving comparably high values of rotary acceleration. In case that one of the two shafts comprises an annular gap for accommodating a hollow-shaft section of the other shaft, it is advantageous with respect to achieving rotary accelerations as high as possible if one cage winding each is provided on either side of the hollow-shaft section of the one shaft accommodated in the annular gap of the other shaft.
Furthermore, for achieving rotary accelerations as high as possible, it is advantageous if the conductor windings are designed to be waveguides or supra-conductors. Here, for cooling the waveguides, it is beneficial if a cooling medium flows through the waveguides.
In order to generate rotary oscillations, wherein the two shafts are pivoted relative to each other by up to ±20°, it is advantageous if the conductors, to which current has been variably applied, are divided into several segments on the shaft, said segments being arranged circumferentially in an even manner and preferably being four, wherein the sections between the conductor segments do not comprise conductors.
In order to achieve a deflection of the two shafts relative to each other in both directions, it is advantageous according to the inventive method if the conductors are fed with alternating current.
Tests have shown that it is advantageous for achieving the desired rotary accelerations of from 10,0000 rad/s2 to 40,000 rad/s2 if alternating current at a frequency of between 0 and 4,000 Hz, preferably of substantially 400 Hz, is applied to the conductors. Likewise, it is beneficial for achieving the desired rotary accelerations if a current strength of between 300 and 2,000 amp, preferably of from 400 to 1800 amp, and a voltage of between 30 and 250 volts, preferably of from 50 to 190 volts, are applied variably to the conductors.
For checking an output-shaft-connected test specimen with regard to its stress capacity on rotary oscillations, it is advantageous if during the application of current to the conductors and during superposition of the magnet field resulting therefrom, the two shafts oscillate relative to each other at a frequency of from 0 to 4,000 Hz, preferably of substantially 400 Hz.
In the following, the invention will be explained in even more detail by way of preferred embodiments illustrated in the drawings, yet without being restricted thereto.